Td5 Thermostat Guide
The Td5 cooling system is a bit different to many engines. Understanding how the thermostat and bypass system works helps you diagnose slow warm up, poor heater performance and overheating.
Opens at: 82°C
Fully open: ~96°C
Pressure cap relief: 1.4 bar
Common symptoms: slow warm up, weak heater, wandering gauge, overheating under load
Td5 Cooling System Pressure
The Td5 engine operates a pressurised cooling system, using pressure to increase the boiling point.
The expansion tank cap has a pressure relief valve which operates if cooling system pressure exceeds 1.4 bar.
Because the system is pressurised, any leak can compromise cooling ability by lowering the boiling point, risking coolant boiling up.
Td5 Thermostat Opening Temperatures
The Td5 thermostat is designed to start opening at 82°C and be fully open at around 96°C.
Ports and Valves
The Td5 thermostat has three ports connecting it to the radiator bottom hose, top hose and coolant pump.
There are two valves incorporated in the thermostat.
- The main valve
- The bypass valve
Main Valve Operation
The main valve maintains optimum coolant temperature and assists in engine warm up.
It remains closed at temperatures below approximately 82°C.
Once temperature increases, the thermostat starts to open and will be fully open at around 96°C.
When fully open, full coolant flow is directed through the radiator.
Bypass Valve Operation
The bypass valve is held closed by a spring which assists in engine warm up.
When the main valve is closed and engine speed is below 1500 RPM, coolant flow does not have the force to open the bypass valve.
When engine speed increases above 1500 RPM, pump pressure opens the bypass valve to limit pressure in the heater circuit.
The valve modulates to provide strong flow through the heater matrix while allowing excess coolant into the bypass circuit for engine cooling.
Essentially the bypass valve is designed to lose heat into the heater circuit rather than throwing it away.
Common Symptoms of a Faulty Thermostat
- Engine takes a long time to reach temperature.
- Poor heater output until driving harder.
- Temperature gauge wanders up and down.
- Overheating under load or in traffic.
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